igneous

Basalt River Cobble

Basalt (Mafic Extrusive Igneous Rock)

Hardness: 6 (Mohs scale); Color: Gray to dark gray; Luster: Dull or earthy; Crystal structure: Aphanitic (fine-grained); Cleavage: None/Conchoidal fracture; Specific gravity: 2.8 - 3.0.

Hardness: 6 (Mohs scale)
Color: Gray to dark gray
Luster: Dull or earthy

Identified April 4, 2026More igneous →

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Physical properties

Hardness: 6 (Mohs scale); Color: Gray to dark gray; Luster: Dull or earthy; Crystal structure: Aphanitic (fine-grained); Cleavage: None/Conchoidal fracture; Specific gravity: 2.8 - 3.0.

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. This specific specimen has been smoothed and rounded by water erosion in a river or coastal environment.

Uses & applications

Used extensively in construction as aggregate, in road base, for decorative landscaping stones (river rocks), and crushed for railroad ballast. High-quality basalt is also used for cast-basalt liners.

Geological facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also found on the Moon, Mars, and Venus.

Field identification & locations

Identify by its characteristic heavy weight for its size, dark gray color, and fine-grained texture lacking visible crystals. Commonly found in volcanic regions and stream beds.

Other igneous specimens

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

mineral

Dark Igneous Rock (Likely Basalt or Diabase)

Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

Igneous

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

mineral

Magnetite

Magnetite (Fe3O4)

mineral